Fasting Before a Blood Test

The simple version

Water is fine. Coffee and tea are not. Chewing gum and mints break the fast. 'Overnight' means 8–12 hours. Most cholesterol and glucose tests need fasting. Most other panels don't. Take your usual medications with water unless we've specifically told you otherwise.

What actually changes with fasting

Triglycerides

This is the main reason lipid panels require fasting. Triglyceride levels rise significantly after a meal containing fat — sometimes 2–3x the fasting value — and stay elevated for 8–10 hours. The standard 12-hour overnight fast is enough to return them to baseline. Total cholesterol, LDL, and HDL change much less post-meal (5–10% at most), which is why some labs now offer 'non-fasting lipid panels' if triglycerides aren't the focus.

Glucose

Eating raises glucose for 2–4 hours. For fasting glucose specifically, 8–10 hours is sufficient. If you're being screened for type 2 diabetes, fasting glucose plus HbA1c gives a more complete picture than either alone.

HbA1c

HbA1c is glycated haemoglobin — it reflects average glucose over the previous 8–12 weeks. Eating an hour beforehand doesn't change it. No fasting required. We often combine HbA1c with fasting glucose for new diabetes workups.

Thyroid (TSH, T3, T4)

Diurnal variation matters more than fasting. TSH is highest in the early morning, lowest in late afternoon. We recommend morning draws for consistency between repeat tests, but you don't need to fast.

Iron studies (ferritin, transferrin, iron saturation)

Iron supplements taken in the 24 hours before the test can transiently elevate serum iron. Stop iron supplements for 24 hours before testing if you want accurate iron studies. No food fasting required.

What about water?

Water doesn't affect any blood test. Drink it freely — before and during the wait. Well-hydrated patients have easier, faster venous draws. Dehydrated patients have collapsed veins that take three attempts and a butterfly needle to access.

The grey areas

Black coffee technically meets a strict 'no calorie intake' fasting definition, but caffeine transiently affects glucose, triglycerides, and some hormone markers. Our position is no coffee for 12 hours before any fasting panel.

Chewing gum, even sugar-free, stimulates digestion (the cephalic phase of digestion responds to chewing alone). Skip it.

Smoking elevates cortisol and some inflammatory markers. If you smoke, try to avoid for 1 hour before the test, and mention it so we can interpret results in context.

Vigorous exercise within 12 hours can elevate creatine kinase (CK) and some liver enzymes. If you've done heavy training the day before, mention it.

Medications: take them, mostly

Skipping blood pressure medication, thyroid medication, anti-epileptics, or asthma inhalers to 'fast' is dangerous and unnecessary. Take all your usual medications with a sip of water. The only exceptions are medications where we've specifically told you to skip (most commonly iron supplements for iron studies, or in some testosterone monitoring contexts).

What we'll do at the appointment

You'll be asked to confirm when you last ate and what (if anything) you've drunk that morning. We then draw to UKAS-accredited UK labs. Routine panels return in 24–48 hours. A pharmacist consultation is included so the result conversation actually means something — we won't just hand you a PDF and walk away.